Page 105 - Canadian BC Science 9

Rules for Writing the Formulas of Ionic Compounds
In an ionic compound, the positive charges balance the negative charges. You can use this balance to find the ratio of positive ions to negative ions. Then use the ratio to write subscripts in the formula. Table 3.4 gives the rules and two examples.
Notice that the final formula must represent the smallest whole number ratio. For example, Sn4􏰀 combining with O2􏰁 is written SnO2 and not Sn2O4.
Table3.4 RulesforWritingFormulasofIonicCompounds Containing Two Elements
from the prefix “sub-,” which means below, and “script,” meaning to write. Subscripts are used in the formulas of ionic compounds to show the relative amounts of each ion.
Word Connect
Steps for Writing the Formula
1. Identify each ion and its charge.
2. Determine the total charges needed to balance positive with negative.
3. Note the ratio of positive ions to negative ions.
4. Use subscripts to write the formula. A “1” is not shown in the subscripts.
Zn3N2
Examples
aluminum chloride
aluminum: Al3􏰀 chloride: Cl􏰁
Al3􏰀 : = 􏰀3 Cl􏰁: 􏰁1 􏰁1 􏰁1 = 􏰁3
1 Al3􏰀 ion for every 3 Cl􏰁 ions
AlCl3
zinc nitride
zinc: Zn2􏰀 nitride: N3􏰁
Zn2+: 􏰀2 􏰀2 􏰀2 = 􏰀6 N3􏰁: 􏰁3􏰁3 =􏰁6
3 Zn2􏰀 ions for every 2 N3􏰁 ions
Practice Problems
1. Write the formulas of the compounds containing the following ions.
(a) Li􏰀 with Cl􏰁 (b) Ca2􏰀 with F􏰁 (c) Na􏰀 with S2􏰁
(d) Ca2􏰀 with S2􏰁 (e) Al3􏰀 with O2􏰁 (f) Al3􏰀 with N3􏰁
2. Write the formulas of the following compounds.
(a) lithium fluoride
(b) silver sulphide
(c) magnesium chloride
(d) zinc oxide
(e) lithium oxide
(f) aluminum iodide
(g) barium phosphide
(h) aluminum phosphide (i) rubidium selenide
(j) strontium nitride
(k) cesium sulphide
(l) sodium nitride (m) zinc phosphide (n) calcium oxide
Answers provided on page 509
Chapter 3 Elements combine to form compounds. • MHR 87
The word “subscript” comes
Suggested Activity
Think About It 3-2B on page 93